Why can I not edit object texture nodes?
The object texture list shown in the node editor is generated from textures used by Blender Internal render engine materials. A material and one of it's textures needs to be selected for this list to show up even though every available texture will then be listed.

These texture slots are only accessible when the Blender Internal render engine is active.

This means that the texture slots need to be created while Blender Render is active, if Cycles is chosen after the slots are populated then the list will still be visible in Cycles, though they will have no use for the material. Materials created while Cycles is active will not have any material slots and will not show the list in the node editor.